Chapter 13

# Chapter 13 - The Second Law of Thermodynamics When do RA...

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The Second Law of Thermodynamics – When do Reactions Occur Spontaneously? A spontaneous reaction is one that is possible under the specified set of conditions (e.g. temperature and pressure). Although thermodynamics can predict if a reaction CAN occur it does not tell us how fast that process will occur. This is determined by the KINETICS. To understand how to predict if a reaction will occur we need to introduce two new state functions, entropy (S) and Gibbs free energy (G). Under conditions of constant temperature and pressure the change in the Gibbs Free Energy (G) during a reaction is what determines if a reaction is spontaneous.

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Spontaneous Processes When a bullet hits a steel plate its kinetic energy is converted into heat and metal droplets spray in all directions. The reverse process is impossible.
Spontaneous Processes - Contd We measure heat flowing from a hot body to a cold one when they are brought into thermal contact, but we never detect heat flowing spontaneously In the opposite direction. We observe a gas expanding into a region of lower pressure, but we never see the reverse process, a gas compressing itself spontaneously into a small part of its container. We place a drop of red ink into a beaker of water and watch the color spread by diffusion of the ink particles until the water is uniformly pink, but we never see the ink spontaneously reorganize as a small red drop in a volume of otherwise colorless water. We place 10 g sucrose (ordinary table sugar) in a beaker and add 100 mL water at 80 °C. The sucrose dissolves to form a uniform solution. We never observe the spontaneous reappearance of a mound of sucrose at the bottom of a beaker of water. 5. We open a container of acetone on the laboratory bench. We detect the aroma of acetone because some of the molecules have evaporated from the liquid and then diffused through the atmosphere to our position. We never observe the molecules retreating spontaneously back into the

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Spontaneous Chemical Reactions Oxidation of many metals (e.g. copper) Reaction of sodium with water
The Thermodynamic Universe Collectively, the system and the surroundings are described as the thermodynamic universe The total amount of energy, volume and matter in the thermodynamic universe is fixed. In a spontaneous process, these quantities are redistributed between the system and the surroundings. The key to understanding if a process will be spontaneous or not is to consider the change in entropy (S) of the thermodynamic universe.

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Entropy What makes a process spontaneous? Consider the simple expansion of a gas into a vacuum. Why does the gas expand into the vacuum?
Statistics Even if there are only 4 molecules then there is only a 1 in 16 chance that they will all be on one side of the container. For one mole of a gas the

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## This note was uploaded on 01/20/2012 for the course CHEM 030.101 taught by Professor Draper during the Fall '08 term at Johns Hopkins.

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Chapter 13 - The Second Law of Thermodynamics When do RA...

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